Pulsed infrared lasers that produce radiation within the wavelength range of approximately 1.5 μm to 2.2 μm are particularly useful for minimally invasive fragmentation of concretions that form within tissues and organs, owing to the relatively strong absorption by urinary and biliary calculi, and the ability to deliver said wavelengths in relatively high energy pulses by means of standard silica clad, silica core optical fiber. These same wavelengths, produced by both pulse and CW mode lasers, also find utility for ablating or vaporizing soft tissues.
Surgical techniques using infrared laser energy delivered by silica optical fiber are also hindered by strong absorption by water. As described in Isner, et al., in “Mechanism of laser ablation in an absorbing fluid field”, Lasers Surg. Med., 8(1988): 543-554, as the ‘Moses Effect’, and often referred to as the ‘Moses bubble’, a steam bubble is formed between the emitting face of the optical fiber and the target in aqueous media, the formation and maintenance of which consumes a substantial portion of the laser energy provided. Only the laser energy that passes through the preformed steam bubble, without encountering liquid water, imparts the target tissue.
For example, a 200 μm core optical fiber (placed 1 mm away from a kidney stone, in a saline irrigated environment) will produce a Moses bubble with a volume of about 0.1 μl within the first several dozen or hundred microseconds of a holmium laser pulse. Assuming the saline is at physiological temperature (the initial temperature affects the result minimally) almost 0.4 J is required to vaporize this volume or water. A 200 μm core laser lithotripsy fiber is typically used with laser average power settings at or under 20 W, where individual laser pulse energies range from around 0.45 J to 2.0 J, such that 20% to 90% of each pulse may be consumed simply in forming the bubble. Larger diameter fibers delivering higher energy pulses do not fare much better, and some fare worse, given that the volume of saline vaporized essentially squares with the fiber diameter.
Estimating losses due to boiling saline (with a 1 mm fiber to target separation and assuming full divergence at the maximum fiber NA, within pure water), a standard 273 μm core fiber loses about 0.5 J, a 365 μm core fiber loses almost 0.75 J, and 550 μm core fiber is diminished by about 1.25 J where a 910 μm core fiber loses about 2.6 J, just to create the steam bubble. The maximum laser pulse energy is limited for most holmium lasers to 5 J although the latest models offer a bit larger pulse energies. Once a Moses bubble is formed, the fiber tip is operating within what is essentially an air environment and the central portion of the beam passes to the target virtually unimpeded; the refractive index elevation and absorptivity due to steam have only minor effect. Some portion of the energy at the phase boundary of the bubble is consumed in maintaining the Moses bubble for the remainder of the pulse duration which ranges from 250 μs to 350 μs for typical holmium lasers and up to 700 μs and longer in some models. The higher angle energy is the least effective in ablating tissue, but there are thermal degradation modes for stones that are aided by even low quality light, e.g. spontaneous fracture due to differential thermal expansion in the polycrystalline matrix. As a general rule one prefers that the laser energy delivered within the confines of the endosurgical field is used to do work rather than boil water.
Accordingly, standard practice in laser lithotripsy calls for the fiber tip to be held in direct contact with the calculus, thereby minimizing energy lost to boiling water. Such intimate contact is virtually impossible to maintain for the duration of surgery, however, and there may be undesirable consequences to this practice as well. Fibers held in contact with stones appear to degrade more rapidly in a failure mode that has been coined “burn back” in the field of laser ureteroscopy. Urological surgeons routinely must remove small core fibers from the ureteroscope to reprocess the output tip during a surgical session or suffer dramatically reduced coupling efficiency via a severely damaged fiber tip. The consensus in the field is that tip damage is a result of physical impacts by particulate calculi ejected during the stone ablation process.
A similar problem exists for side fire fibers in soft tissue ablation, but with more serious consequences; if a side fire fiber is used in direct contact with soft tissue in holmium surgery, the fiber protective cap becomes frosted and pitted and must often be discarded and replaced in order to complete the surgery. Side fire fibers cannot be reprocessed intraoperatively. Providing for side fire fibers to be used with fixed separation from tissue and shielding the fiber from contact have been focuses of prior art in side fire fiber technology.
U.S. Pat. No. 5,454,807 (Lennox, et al.) teaches the provision of a coaxial coolant flow, gaseous or liquid, for prevention of surface tissue damage to irradiated tissue for the stated reason of permitting the application of more laser energy to underlying tissues to improve, for example, exogenous chromophore activation in photodynamic therapy (PDT).
U.S. Pat. No. 5,685,824 (Takei) teaches a “prostascope” provisioned with a standard working channel to accept an optical fiber and deliver irrigant, but where a reflector is positioned within a side opening for redirecting laser energy laterally with respect to the scope longitudinal axis: a side firing scope, if you will.
U.S. Pat. No. 6,246,817 (Griffin) teaches a reduced divergence output fiber that is ferrule reinforced for improved longevity in cryptoscopic laser lithotripsy.
U.S. Pat. No. 6,802,838 (Loeb, et al.) teaches a side firing fiber housed within a nested, dual coaxial lumen device whereby cooling fluid is passed about the side fire fiber within the central lumen and exits a common port and where coolant fluid and debris are evacuated via the same port through the second, surrounding lumen.
U.S. Pat. No. 6,953,458 (Loeb) teaches a coaxial coolant channel about an axial fiber where a gas and laser energy exit a common port, where the channel may be angled for access to orthogonally situated tissues, the gas being utilized to produce a substantially fluid free optical path for the laser radiation to reach target tissues.
U.S. Pat. No. 7,359,601 (Loeb) is a continuation-in-part of Loeb '458 teaching adaptations for standard side-firing fibers as well as bare, bevel-tipped side fire fibers.
U.S. Pat. No. 7,909,817 (Griffin, et al.) discloses a dual cap side fire fiber where the side fire function is provided within the inner, thin walled cap and the physical protection function is performed by the thicker, outer cap with cooling provided by irrigation fluid flow between the two caps during surgery. This technology will not function in the infrared region that is of interest to the current invention (due to Moses bubble formation within the confined space interfering with continuous fluid flows) but at 532 nm on the GreenLight XPS™ laser, it is the most widely used side fire fiber to date: American Medical Systems' MoXy™ fiber (AMS is currently a part of Boston Scientific and owns the preceding trademarks). U.S. Pat. No. 8,529,561 (Griffin, et al.) is a divisional of Griffin '817 describing methods for disrupting laminar flow within the annular, coaxial fluidic conduit. U. S. Pat. Pub. No. 2014/0074072 (Griffin, et al.) is a continuation-in-part of Griffin '561, teaching rotation of the outer, secondary capsule during surgery.
U.S. Pat. Nos. 8,932,289, 9,005,195 and 9,017,324 and a couple of dozen published applications (Mayse, et al.) teach cryogenically cooled tissue ablation devices for treatment of chronic obstructive pulmonary disease with various forms of energy, preferably radio frequency energy, but including laser energy, where cryogenic coolant is delivered via a lumen to a balloon, within which, or about which, resides the energy delivery electrode or presumably an optical fiber or fibers.
Coaxial cooling of fiber tips with gas flow has a long history in the laser surgical fiber design discipline. As early as the 1980s, “gas-cooled” Nd:YAG fibers were produced and sold by Lasersonics, US Surgical, and others for ‘open surgery’ applications, (typically non-endoscopic and no irrigation) such as found in the ear, nose and throat (ENT) specialization, where a circumferential sheathe of gas protected fiber output tips operating in air from blood and tissue ejecta contamination. A niche market remains for these fibers even today, e.g. the Gas/Liquid-Cooled Fiber sold by LightGuideOptics (Germany) and the model DSLF-60 Gas Sheathed Laser Fiber made by Laser Peripherals (Minnesota). Other applications for passing gases and liquids across fiber surfaces or over tissues also appear in the prior art, e.g. cooling tissue in cosmetic and other non-ablative laser procedures to permit more laser interaction with target chromophores (tattoo ink, spider veins, port wine stains and activation drugs for PDT, where the coolant is provided coaxially or by some other means.
In endosurgical applications of lasers, most of the cooled fiber prior art is concerned with side firing fibers for laser vaporization of the prostate or axial firing fibers for prostate enucleation. Loeb, et al. '838 teaches a side fire fiber that is housed within the lumen of a needle having a side port for the laser radiation to exit. Sterile irrigation fluid flows within the needle in an annulus about the central cylinder occupied by the side fire fiber and exits through the same port within the needle as the laser energy. This apparatus is housed within the lumen of a second tube that provides communication between a vacuum source and the area immediately outside of the needle exit port. In recovering the added volume of irrigation fluid from the surgical site, more fluid flow is possible and the fiber may be used in surgical procedures with closer confines and far smaller fluid reservoirs, such as laser discectomy. '838 further teaches that the provided suction removes suspended tissue debris from the surgical field: debris that could otherwise adhere to the side firing fiber transmissive surface. The opposite may be experienced in practice; suction about the laser output port pulls floating debris from the adjacent surgical site preferentially toward the fiber transmissive surface, amplifying tissue adhesion and ultimately leading to catastrophic fiber failure.
Griffin '817 addresses the issues of fiber damage and loss of laser energy to Moses bubble formation with a two-fold strategy: reduction of the volume of water in the column between the fiber output and the surgical target via a terminal, lens-ended up-taper for reduced divergence and a hermetically fused silica ferrule about the up-taper for increased mass at the fiber to target contact. A problem with the art taught in '817 is that it is incompatible with the size limitations for applications of the most damage susceptible fibers. The divergence reduction strategy requires the diameter of the fiber to be locally substantially increased, over a length of a centimeter or more. The delicate taper section (bare fiber) requires protection by a surrounding silica ferrule of even more substantial diameter and greater length than the bare taper segment to house the taper. (Boston Scientific opted to forego the use of a protective silica ferrule about the bare fiber segments in AccuTrac™ and Flexiva™ TracTip fibers and, instead, re-coats the bare fiber with a sacrificial polymer substance. The resulting fibers are compatible with ureteroscope working channels but the longevity of the tips is little improved (Kronenberg, et al., in “Lithotripsy performance of specially designed laser fiber tips”, J Urol., 195-5(2015): 1606-1612).
A fiber for use in the infrared where water and blood strongly absorb the laser energy is taught by Loeb ('458), where gas is passed about a fiber for the purpose of displacing interfering fluids as depicted in FIG. 1. Per standard practice, the optical fiber 35 resides within the lumen of a sheath 30 having a bore that is considerably larger than the fiber 35 and where the end of sheath 15 and the output face of the fiber 10 terminate substantially within a common plane. A gas supply 25 is provided to the sheath lumen via a T or Y fitting 20 or the like and flow is adjusted such that the output face 10 of the fiber may be held in contact or near contact with tissue, while a cloak of gas 45 displaces the irrigation fluid (or other fluid, such as blood) at the immediate surgical site. As such, the emitted laser radiation 40 passes to the tissue via an optical path substantially free of absorbing irrigant or blood.
A problem with the invention taught by Loeb (FIG. 1) is that, unless the sheath forms a gas-tight or nearly gas-tight seal with the target tissue, considerable gas flow is required to prevent intrusion by interfering fluids and, if the flow is sufficient to displace the inflow of fluids at the surgical site, such flow also displaces the irrigation volume as a whole, emptying the surgical site. Reducing the irrigation presence about the fiber at the surgical site also reduces some of the fluid flow function: cooling and cleaning the fiber tip to forestall optical and physical damage. Additionally, the rigid or semi-rigid sheath made of steel, rigid polymer, shape-memory alloy, and like materials, as taught by Loeb is opaque or at best translucent and, as such, obscures visualization of the surgical effect of the laser fiber such that any seal between the tissue and sheath, even a leaky seal, must periodically be broken to access surgical progress and for surveillance of routine complications such as open and bleeding arteries.
Loeb extends the application of air-sheathing to side firing fibers in '601, teaching bare, bevel-tipped side fire fibers—absent the ubiquitous transparent protective cap—and claiming that the low refractive index of the sheathing gas (carbon dioxide under continuous flow) is sufficient for preserving conditions required for total internal reflection (TIR) according to Snell's law, even where the fiber is used in an aqueous environment. While flows of sufficient volume and pressure to exclude moisture from contaminating the refractive index barrier necessary for TIR is theoretically feasible, such volumes and pressures are not compatible with the confined and tissue-bound space of the endosurgical environment. The carbon dioxide (or other “biocompatible gas”) flow necessary to continuously displace the surgical irrigant from the side fire optical path is problematic as no mechanism for removing the deployed gas is provided.
Simply relying upon the gas to bubble past the irrigation flow within the cystoscope working channel is likely inadequate. Surgical interventions can take hours: e.g. for relief of the symptoms of benign prostatic hyperplasia (BPH). Where the surgical site is the prostatic urethra, adjacent to the urinary bladder, the pressures within the urethra may rise sufficient to open the interior sphincter, inflating the bladder. Further flow then fills the ureters and ultimately the kidneys where fatal consequences due to gas perfusion into the extensive capillary bed are possible. Perfusion into capillaries exposed by the surgery itself could be problematic enough on its own. While some portion of the optical path may be free of water during some portion of energy delivery events with sheath gas flows compatible with BPH surgery, total displacement of irrigant from the optical path is improbable. Were Loeb's inventions to be adapted to axial firing fibers intended for use in the kidney—the focus of the invention disclosed herein—gas perfusion into the capillary bed of the kidney would be extremely problematic.
Two prior art patents and the patent application by Griffin, et al. disclose numerous embodiments for coaxial cooling of side fire fibers that avoid problems of earlier designs, as manifest in the commercial success of an embodiment of the device sold for the vaporization of hyperplastic prostate tissue with green laser light. Green light resides within the biological or optical window, where water is essentially transparent such that there is no benefit to excluding irrigation fluids from the optical path from fiber to tissue and Griffin makes no effort to do so.
The plethora of patent applications and issued patents by Mayse, et al., deal with cooling energy delivery devices for use in an air environment—the lungs—and while a mention of the potential for use of a laser as an energy source does appear, the overriding concern of these disclosures is with cooling about, or concurrent with, application of radio frequency energy via an electrode (antenna) and has no real bearing on the invention disclosed herein.